Generally speaking, automatic fast filters are provided with motorized mechanical systems for ensuring their cleaning and are always cleaned by back wash.
These mechanical systems need a pluri-metallic construction, thus corrosive, and the cleaning by back wash induces a risk of tacking and/or felting of the filtering elements when the water to be filtered contains fibers.
Swiss patents Nos. 516.331, 532.409, 597.894 and 678.400 in the name of the applicant provide solutions to the above mentioned disadvantages as they describe filters with cleaning by direct scavenging of the filtering surface. Patents Nos. 516.331 and 532.409 disclose a simple mono-element filter with laminar cleaning on a tubular filtering element, in which a current of untreated water is flushed at the moment of cleaning by opening of a flush valve normally closed. These two patents also explain the evolution of a virtual filtration ring developing from down-stream to up-stream according to the stage of clogging of the filter. This theory allows to design a multielement filter with several finenesses, described in patent No. 597.894 in which each filtering element is extended by a knee crossing through the body of the filter by a branching and a flushing valve, normally closed. All these individual flush branchings are then connected to a flushing collector. These filters work perfectly, but they are expensive to manufacture, because of the multiplication of the flush branchings and of the valves.
Patent 678.400 allows to lower the manufacturing cost of such filters by introducing a turbo-distributor which brings into contact each filtering element one after the other with one single flush outlet. This allows to have only one flush branching and only one automatic valve (instead of 6 or 8), to suppress the flushing collector and to simplify the installation and the programming of such a filter. Nevertheless the latter presents the following disadvantages:
The rotor of the turbo-distributor is an expensive part relatively complex to manufacture. In the presence of untreated water charged with impurities, one observes a wear of the moveable parts, such as the rotor of the turbo distributor. In certain extreme conditions, the rotor may grip, which needs a disassembling of the filter and thus the temporary closure of the water pipe of the filter.